Recovery of lactic acid values from a meso-lactide stream

ABSTRACT

Lactic acid equivalents are recovered from a starting lactide stream by catalytically racemizing a portion of the lactide in the stream at a temperature of 180° C. or below. This increases the proportion of two species of lactide (i.e., at least two of S,S-, R,R- or meso-lactide) at the expense of the third species. The racemized mixture so obtained can be separated to recover some or all of one or more of the lactide species from the remaining lactide species, by a process such as melt crystallization or distillation. Impurities in the starting lactide stream usually are retained mostly in the remaining meso-lactide, so a highly purified S,S- and/or R,R-lactide stream can be produced in this manner. Such a purified S,S- and R,R-lactide stream is suitable for polymerization to form a polylactide.

This application claims priority from U.S. Provisional PatentApplication No. 61/159,929, filed 13 Mar. 2009.

This invention relates to methods for making lactide and poly(lacticacid) (polylactide) resins.

Lactide is a monomer that is polymerized to produce polylactide resins.Processes that are suitable for large-scale production of polymer gradelactide from lactic acid are described, for example, in U.S. Pat. Nos.5,247,058, 5,258,488, 5,357,035, 5,338,822, 6,005,067, 6,277,951 and6,326,458. The processes described in these patents generally involvepolymerizing lactic acid to form a low molecular weight polymer, andthen depolymerizing the low molecular weight polymer. Thedepolymerization step produces lactide. The lactide is then purified toseparate it from impurities that may include, for example, water,residual lactic acid, linear lactic acid oligomers, and otherimpurities.

Lactic acid is a molecule with one chiral center, and so it exists astwo optical isomers, the so-called R- (or D) enantiomer and the S- (orL) enantiomer. The lactic acid that is used as the raw material forproducing lactide is usually of very high optical purity. As the lacticacid passes through the steps of forming the lactide, it is exposed toelevated temperatures and some of the lactic units convert from oneoptical isomer to another, to form a mixture of the R- andS-enantiomers. The process of converting one optical isomer of anorganic compound to another is known as “racemization”.

Lactide corresponds to the condensation product of two molecules oflactic acid to form a 3,6-dimethyl-1,4-dioxane-2,5-dione. Lactidetherefore can be considered as being made up of two “lactic units”, eachof which has the formula C₃H₄O₂. Each lactic unit in a lactide moleculecontains one chiral center and exists in either the R- or the S-form. Alactide molecule can take one of three forms:3S,6S-3,6-dimethyl-1,4-dioxane-2,5-dione (S,S-lactide),3R,6R-3,6-dimethyl-1,4-dioxane-2,5-dione (R,R-lactide), or3R,6S-3,6-dimethyl-1,4-dioxane-2,5-dione (R,S-lactide or meso-lactide).These have the following structures:

S,S-lactide and R,R-lactide are a pair of enantiomers, whilemeso-lactide is a diastereomer.

Most lactic acid that is produced commercially is the S-enantiomer. WhenS-lactic acid is converted to lactide, the major product is thereforeS,S-lactide. However, because some of the S-lactic acid racemizes toR-lactic acid, some R,R-lactide and some meso-lactide are also produced.The ratios of S,S-lactide, meso-lactide and R,R-lactide that areproduced when the low molecular weight polymer depolymerizes can beestimated as:

-   -   S,S-lactide mole fraction≈(F_(S))²    -   R,R-lactide mole fraction≈(F_(R))²    -   Meso-lactide mole fraction≈2F_(R)F_(S)        wherein F_(R) is the mole fraction of R-lactic enantiomer and        F_(S) is the mole fraction of S-lactic enantiomer in the low        molecular weight polymer which is depolymerized to form the        lactide. There is a small kinetic bias towards making        S,S-lactide and R,R-lactide, and so the foregoing estimates may        slightly overestimate the amount of meso-lactide that is        produced. The meso-lactide fraction will be larger than the        R,R-lactide fraction when F_(R) is smaller than F_(S), as is        normally the case. The R,R-lactide fraction is often quite        small.

It is usually necessary to separate meso-lactide from the rest of thelactide stream. There are two reasons for this. One has to do withcontrolling the proportion of the R-lactic acid units in the lactidestream that is taken into the polymerization step. It is important tocontrol the ratio of S- and R-lactic units in the lactide stream, asthat ratio can significantly affect the crystalline properties of apolylactide made by polymerizing the stream. Removing meso-lactide fromlactide stream has the effect of reducing the proportion of R-lacticacid units, which leads to the production of a more crystalline grade ofpolylactide.

The second reason has to do with removing certain impurities from thelactide. Certain common separation methods, such as distillation andmelt crystallization, tend to concentrate impurities in the meso-lactidestream, thereby further purifying the S,S- and R,R-lactide stream.

The separated meso-lactide stream contains lactic acid equivalents,which are valuable if they can be recovered. However, a significantamount of clean-up is needed because impurities tend to becomeconcentrated in that stream. It has been difficult to remove certainimpurities from this stream in an economical way. Distillation methodsare ineffective at a commercial scale, because at least some of theimpurities have volatilities very close to that of meso-lactide. Anotherproblem is that the meso-lactide stream is very optically impure becauseit contains high proportions of both S- and R-lactic acid units. It canbe blended with an S,S-lactide stream in small proportions (less thanabout 15% by weight meso-lactide) to produce a semi-crystallinepolylactide, but the meso-lactide stream only produces amorphouspolylactide grades if polymerized by itself or in higher proportions. Asa result of these problems, much or all of the meso-lactide is usuallydiscarded or used in other, non-polymer applications which have lowervalue. These problems reduce overall yields and increase the overallcost of the process.

In cases such as just described, it would be desirable to reduce theseyield losses by recovering lactic acid equivalents from the meso-lactidestream in a form that can be used to make polylactides. It is furtherdesirable to recover those lactic acid equivalents mainly in the form oflactide, rather than in the form of lactic acid or linear lactic acidoligomers.

To state the problem more generally, there sometimes exist situations inwhich an available lactide stream contains a diastereomeric and/orenantiomeric composition that is different from what is needed for aparticular application. The most usual case is the one just described,in which a meso-lactide stream is available and S,S-lactide and/orR,R-lactide are what is needed. However, other cases can exist. Forexample, an available lactide stream may contain predominantly S,S- orR-R-lactide, when a stream rich in meso-lactide is required. In anotherpossible scenario, a predominantly S,S-stream may be available whileR,R-lactide is required, or vice versa. In each of these situations, itis desired to extract as much of the wanted lactide product as possiblefrom the available lactide stream, and so reduce yield losses.

Another desired outcome would be to obtain the desired lactide productin a somewhat purified state. As discussed more fully below, in certainmanufacturing processes in which meso-lactide is separated fromS,S-lactide and/or R,R-lactide, many of the impurities tend to becomeconcentrated in the meso-lactide stream. For example, distillation andmelt crystallization methods for removing meso-lactide from the otherforms will preferentially leave impurities with the meso-lactide, andproduce a stream of S,S-lactide and/or R,R-lactide that is relativelyclean. The impurities that remain with the meso-lactide are oftendifficult to separate from it. It would be desirable to provide a methodin which lactide values can be recovered from a meso-lactide stream thatis contaminated with these impurities, and in which the recoveredlactide values are relatively free of those impurities.

Although lactic acid can easily racemize, and lactide units within alactic acid oligomer can isomerize, lactide itself is not known toracemize under any reasonable conditions. Tsukegi et al., in“Racemization behavior of L,L-lactide during heating”, Polym.Degradation and Stability 92 (2007) 552-559, report that racemization ofL,L-lactide to DD-lactide and meso-lactide can occur. However, theracemization proceeds very slowly at temperatures less than 270° C., andat those temperatures large amounts of oligomers form.

This invention is in one aspect a process for recovering lactic acidvalues from a starting lactide composition comprising a) subjecting astarting lactide composition to a temperature of up to 180° C. in thepresence of a racemization catalyst for a time sufficient to racemize atleast a portion of the lactide in the starting lactide composition toform a racemized lactide mixture that contains meso-lactide, S,S-lactideand R,R-lactide in relative proportions different than in the startinglactide composition.

The catalytic racemization permits commercially reasonable racemizationrates to be achieved without producing large amounts of ring-openedspecies such as lactic acid or lactic acid oligomers. The process isespecially useful for producing S,S- and R,R-lactide from meso-lactide,but is also useful for producing meso-lactide from S,S- and/orR,R-lactide, S,S-lactide from R,R-lactide, or R,R-lactide fromS,S-lactide, if desired.

A preferred process includes the additional step b) of separating theracemized lactide mixture to obtain at least one lactide product that isenriched in S,S-lactide, R,R-lactide or meso-lactide, or any twothereof, relative to the racemized lactide mixture.

Useful racemization catalysts include a metal carboxylate salt, a metalsulfonate, sulfinate, phosphonate or phosphinate salt, or anon-nucleophilic acyclic or cyclic tertiary amine compound;

For purposes of this invention, the term “racemize” or “racemization”refers simply to a process in which one diastereomeric or enantiomericform of lactide, i.e., S,S-lactide, R,R-lactide or meso-lactide, becomesconverted to another diastereomeric or enantiomeric form of lactide.This includes the case in which meso-lactide isomerizes, at equal rates,to S,S-lactide and R,R-lactide, including in particular a process inwhich meso-lactide produces racemic-lactide. It further includes theconversion of S,S-lactide or R,R-lactide to meso-lactide. Those termsare not intended to mean that the isomerization is continued until achemical equilibrium amongst all the lactide forms is reached, althoughin specific embodiments this occurrence may take place.

“Racemized lactide mixture” is used herein as a shorthand to denote amixture of S,S-, R,R- and meso-lactide that is produced in step a) ofthe process, i.e., by racemization of a starting lactide stream.Similarly, the term “racemized S,S-lactide and R,R-lactide” is used as ashorthand to denote S,S-lactide and R,R-lactide that is produced in stepa) of the process, i.e., by racemization of a starting lactidecomposition. The term “racemized” in this context is not intended tospecify any particular properties or composition of the specifiedmaterial, other than its source.

“Racemic lactide” refers to an approximately 50/50 mixture of S,S-lacticacid and R,R-lactic which has a melting temperature of approximately127° C.

This process provides at least three main advantages. First, lactic acidequivalents from the starting lactide stream are recoverable mainly inthe form of lactide (i.e., S,S-,R,R- and/or meso-lactide), rather thanas lactic acid or lactic acid oligomers. Little of the starting lactidereacts to form hydrolyzed species such as lactic acid or linear lacticacid oligomers. Instead, the starting lactide is believed to racemizedirectly, without going through a ring-opened intermediate. The lactideproduct that is obtained can be sent directly to polymerization in somecases, and in other cases can be recycled into various places within alactide production process to further purify it if needed. In eithercase, lactic acid equivalents are recovered from the starting lactide inthe form of lactide molecules, and process losses are reduced.

The second advantage is that the amount of unwanted lactide iscorrespondingly reduced. Therefore, a smaller amount of lactide must bediscarded or used in lower-value applications. As described below,lactide that remains after the separation step can in some cases berecycled back into one or more upstream processes.

A third main advantage is impurities tend to become concentrated in thelactide that remains after the racemization and separation step. This isespecially the case when the remaining lactide is rich in meso-lactide.As a result, a lactide product obtained in step b) which is enriched inS,S- and/or R-R-lactide and depleted in meso-lactide, compared to theracemized lactide mixture, often is purified as a result of the process,by which it is meant simply that the concentration of impurities in thatlactide product is less than in the starting lactide composition. Alactide product obtained from step b) of the process, which is enrichedin S,S- and/or R,R-lactide but depleted in meso-lactide, compared to theracemized lactide mixture, often can be polymerized with little or nofurther purification to produce a polylactide resin.

In certain embodiments, the starting lactide composition is produced byseparating a lactide mixture to form a meso-lactide-enriched stream andan S,S- and R,R-lactide stream that is depleted of meso-lactide relativeto the meso-lactide stream. Either of these streams can be used in thisprocess as the starting lactide composition, but themeso-lactide-enriched stream is the preferred starting lactidecomposition. More specifically, in certain embodiments the startinglactide composition is produced by

-   1) forming a low molecular weight poly(lactic acid);-   2) depolymerizing the low molecular weight poly(lactic acid) to form    a crude lactide; and then;-   3) removing meso-lactide from the crude lactide in one or more steps    such that    -   A) a meso-lactide stream is formed; and    -   B) an S,S- and R,R-lactide stream is formed which is depleted of        meso-lactide relative to the meso-lactide stream.        In these embodiments, the meso-lactide stream produced in        step 3) is taken into the racemization process of this        invention.

The step of removing meso-lactide from the crude lactide in step 3)preferably is conducted by performing a fractional distillation on thecrude lactide. In another approach, step 3) is conducted by meltcrystallization, and the meso-lactide stream is produced as a residuestream in the melt crystallization. Other separation methods such assolvent crystallization also can be used. The step of removingmeso-lactide from the crude lactide is preferably performed so thatimpurities in the crude lactide become concentrated in the meso-lactidestream.

In each of the foregoing aspects and specific embodiments, all or aportion of the racemized lactide product formed in step a) of theprocess or recovered in step b) of the process may be polymerized toform a polylactide, with or without further purification and optionallyin a mixture with another source of lactide, such as the S,S- andR,R-lactide stream that is obtained when meso-lactide is separated fromthe crude lactide stream in step 3) above.

Because the starting lactide composition of most interest is believed tobe a meso-lactide composition, the invention will be described in moredetail from that perspective. The process operates in a mannercompletely analogous to that described in more detail below when thestarting lactide composition contains mainly S,S-lactide, mainlyR,R-lactide, or mainly a mixture of S,S-lactide and R,R-lactide.

Various processes for producing lactide are known, and this inventioncan be used in connection with any of those processes, provided that amixture of meso-lactide and at least one of S,S-lactide and R,R-lactideis produced. Generally, these processes start with lactic acid or aderivative such as a lactic acid salt or a lactic acid ester. In aparticularly useful process for producing lactide, lactic acid or aderivative is polymerized to form a low molecular weight poly(lacticacid) which is depolymerized to form lactide. Processes such as theseare described in U.S. Pat. Nos. 5,536,807, 6,310,218 and WO 95/09879.These processes produce a mixture of S,S-lactide, R,R-lactide andmeso-lactide.

The low molecular weight poly(lactic acid) is suitably prepared byforming a concentrated lactic acid or lactic acid derivative stream thatcontains from 60 to 95% by weight lactic acid or lactic acid derivativein water or, less preferably, another solvent. This stream may containsome oligomeric species that form as the concentrated stream isproduced. This starting material is then further concentrated byremoving water (or a lower alcohol in the case of a lactic acid ester)and solvent (if any) in an evaporator. This causes the lactic acid orderivative to condense, eliminating water or a lower alcohol as thecondensation by-product. As this is an equilibrium reaction, the removalof condensation by-products favors the further condensation of thelactic acid or lactic acid derivative. A low molecular weightpoly(lactic acid) formed this way has a molecular weight of up to about5000, preferably from 400 to 3000.

The low molecular weight poly(lactic acid) is then depolymerized bysubjecting it to an elevated temperature and subatmospheric pressure,typically in the presence of a depolymerization catalyst. Conditions aregenerally selected to (1) minimize residence time, as doing so reducesthe amount of racemization that can occur prior to depolymerization, and(2) vaporize lactide that is formed. The depolymerization reaction isusually catalyzed with a tin or other metallic catalyst. Like thepolymerization reaction, the depolymerization is an equilibrium reactionand removal of the lactide as it is formed favors the production ofadditional lactide. Therefore, continuous removal of crude lactide ispreferred. The crude lactide is preferably removed as a vapor. One ormore stabilizers can be present during this step as described in WO95/09879.

The starting material used in the foregoing process is usually of veryhigh optical purity, i.e., one enantiomer is highly predominant. As thelactic acid or derivative passes through the steps of forming thelactide, some of the lactic acid or lactic acid units in oligomerizedlactic acid racemize and a mixture of S- and R-enantiomers forms.

As already described, the crude lactide that is produced as the lowmolecular weight poly(lactic acid) depolymerizes contains S,S-lactide,R,R-lactide and meso-lactide at ratios that are largely but not entirelystatistically determined in accordance with the proportion of the S- andR-enantiomers in the low molecular weight poly(lactic acid). Of primaryinterest to this invention are crude lactide mixtures that contain fromabout 0.5 to about 30%, especially from 2 to 30% by weight meso-lactide(based on the combined weight of lactide in the mixture). The remaininglactide in those mixtures will be predominantly S,S-lactide orpredominantly R,R-lactide. In the usual case, S,S-lactide will be thepredominant species, and R,R-lactide will be non-predominant. Tosimplify the following discussion, it will be assumed that S,S-lactideis the predominant lactide species in the crude lactide. However, theinvention can be practiced equally well using a crude lactide that iseither predominantly S,S-lactide or predominantly R,R-lactide.

The crude lactide formed in the depolymerization step usually contains,in addition to the lactides, impurities such as residual water, somelactic acid (or a lactic acid salt or ester, if used as the startingmaterial), some linear oligomers of lactic acid, and other reactionby-products. Meso-lactide is separated from S,S- and R,R-lactide. Thecrude lactide stream may undergo one or more purification steps prior toor simultaneously with this separation. For example, the crude lactidemay be partially condensed to separate it from more volatile impurities.Alternatively, the crude lactide can be purified by melt crystallizationmethods as described in U.S. Pat. No. 6,310,218. A third approach is todistill off some or all of the impurities that are significantly morevolatile than meso-lactide, such as water, residual lactic acid orlactic ester starting materials, and other small organic compounds. Sucha distillation step can be performed prior to or simultaneously with afractional distillation step in which meso-lactide is separated from theS,S- and R,R-lactide.

Meso-lactide can be separated from the S,S- and R,R-lactide bydistillation, melt crystallization or other suitable processes. Foreconomic reasons, distillation processes are preferred at large scale.The separation creates a meso-lactide stream and an S,S- and R,R-lactidestream.

The S,S- and R,R-lactide stream obtained by separating meso-lactide fromthe crude lactide stream contains essentially all of the S,S- andR,R-lactide that were present in the crude lactide stream, and maycontain some meso-lactide. The meso-lactide composition contains mainlymeso-lactide. For purposes of this invention, a meso-lactide compositionis considered to contain at least 60% by weight of meso-lactide, and maycontain at least 80% or at least 90% by weight of meso-lactide, based onthe total weight of lactide in the composition. It may contain smallquantities of S,S- or R,R-lactide, but these together generallyconstitute no more than about 40%, preferably no more than 20% and evenmore preferably no more than 10% by weight of the lactide content of ameso-lactide composition. Thus, the meso-lactide composition is enrichedin meso-lactide, compared with the S,S- and R,R-lactide stream andcompared with the crude lactide.

It is difficult to separate some of the impurities from lactide in adistillation process because those impurities have volatilities sosimilar to meso-lactide and S,S- or R,R-lactide. Process economics oftendictate against making this additional separation in a distillationcolumn, either because of the cost of the necessary equipment, theimpact on operating rates, or some combination of both. Most if not allof these will remain in either the meso-lactide stream or the S,S- andR,R-lactide stream after the fractional distillation step. Theimpurities tend to become more concentrated in the meso-lactide streamwhen the meso-lactide is separated from the S,S- and R,R-lactide,especially when the separation is performed by a fractionaldistillation. Typically, therefore, the meso-lactide stream is enrichedin impurities, relative to the crude lactide stream and the S,S- andR,R-lactide streams. The S,S- and R,R-lactide streams are greatlydepleted in impurities, relative to the crude lactide stream and themeso-lactide stream.

A starting lactide composition that is taken for racemization (such as ameso-lactide composition as described above) may contain up to about20%, more typically up to about 5% by weight, of impurities (i.e.,materials other than a species of lactide). However, the startinglactide composition should contain no more than 50 milliequivalents/gramof hydroxyl-containing species, and preferably less than 20milliequivalents/gram of hydroxyl-containing species.

A starting lactide composition, such as a meso-lactide stream producedin the manner described above (or by other appropriate process), formsthe starting material that is taken into step a) of the inventiveprocess. The starting lactide composition is racemized in the presenceof a racemization catalyst and at a temperature of up to 180° C., for atime sufficient to racemize a portion of the starting lactide, so thatthe proportions of S,S- R,R- and meso-lactide become changed in thecomposition. When the starting lactide composition is a meso-lactidecomposition, the net effect of the racemization is to convertmeso-lactide into racemized S,S-lactide and R,R-lactide. Thus, anS-lactic unit is converted to an R-lactic unit, or vice versa. When anS-lactic unit of a meso-lactide molecule racemizes, the meso-lactidemolecule is converted to R,R-lactide. Similarly, when an R-lactic unitof a meso-lactide molecule racemizes, a molecule of S,S-lactide isformed.

Meso-lactide, S,S-lactide and R,R-lactide can all racemize under theconditions of the racemization reaction. A molecule of meso-lactide hasa statistically equal chance of racemizing to S,S-lactide orR,R-lactide. As S,S-lactide and R,R-lactide molecules are formed, thesecan also racemize back to meso-lactide (which can racemize again to S,S-or R,R-lactide). Likewise, any S,S- or R,R-lactide in the startingmixture can racemize to meso-lactide. Because the racemization reactionsare random (tending towards a temperature-dependent chemical equilibriumunless one or more of the lactide forms is selectively removed), theproportions of meso-lactide, S,S-lactide and R,R-lactide that arepresent in a racemizing mixture at any given time, assuming no removalof product, will depend on (1) the proportions of meso-, S,S- andR,R-lactide in the starting mixture, (2) the racemization temperature,(3) the type and amount of catalyst and (4) the amount of time themixture is exposed to racemization conditions. Over time, the respectiveproportions of S,S-, meso- and R,R-lactide will shift towards anequilibrium, which is temperature-dependent.

The temperature of the racemization reaction has three primary effects.Racemization rates increase with increasing temperature, and thereforehigher temperatures are favored when a faster reaction rate is needed orwanted. On the other hand, more reaction by-products, especially linearlactic acid oligomers, form as racemization temperatures become higher.In addition, the racemization temperature affects the equilibriumproportions of S,S-, meso- and R,R-lactide that are produced, with theequilibrium shifting towards more meso-lactide production at highertemperatures. At about 160° C., for example, a racemizing lactidemixture will in time reach an equilibrium ratio of about 36% each ofS,S- and R,R-lactide, and about 28% meso-lactide. At 140° C., thisequilibrium ratio is about 38% each of S,S- and R,R-lactide and 24% ofmeso-lactide. At 105-120° C., the equilibrium ratio is about 40-42% ofeach of S,S and R,R-lactide and about 15-20% of meso-lactide. Of course,more S,S- and D,D-lactide can be produced by shifting the equilibrium byremoving S,S- and R,R-lactide as the racemization reaction proceeds.

Therefore, the racemization temperature should be selected in anyparticular case to balance these effects. The racemization temperatureshould be above the melting temperature of meso-lactide, which isapproximately 56° C. More preferably the racemization temperature is atleast 97° C. The racemization temperature is no greater than 180° C.,preferably no greater than 170° C., to avoid producing significantquantities of linear lactic acid oligomers. If too many linear lacticacid oligomers are produced, they must be removed before the S,S- andR,R-lactide can be used in a polymerization reaction, which can resultin a loss of yield, increased purification costs, or both.

In some embodiments, the racemization temperature is above the meltingtemperature (127° C.) of so-called racemic lactide, which is a 50/50mixture of S,S- and R,R-lactide. In those embodiments, an especiallypreferred racemization temperature range is from about 135 to 170° C.,especially about 140 to 160° C.

In other embodiments, the racemization temperature is at or below themelting temperature of racemic lactide. In those cases, a preferredracemization temperature is from 90 to 125° C., especially from 90 to115° C. and preferably from 90 to 100° C. Under these conditions themixture can be directed to a situation where the concentration of R,R-and S,S-lactide at the chemical equilibrium would exceed the solubilitylimit at the chosen temperature. Thus, as meso lactide is converted toR,R- and S,S-lactide, the R,R- and S,S-lactide can reach a concentrationwhere they crystallize out of the solution, often in the form of racemiclactide. In this manner the meso-lactide can be converted directly to acrystallized mixture of R,R- and S,S-lactide without reaching thechemical equilibrium limit.

In still other embodiments, a portion of the racemization step can beconducted at a temperature above 127° C., such as from 135 to 170° C. orfrom 140 to 160° C., to take advantage of faster racemization rates.This temperature may be used, for example, until the meso-lactidecontent in the racemizing mixture drops below, for example, 40% or 30%,at which point the racemization temperature may be reduced to from 90 to125° C., preferably from 90 to 115° C. or from 90 to 100° C. The lowertemperature in the second step shifts the equilibrium away frommeso-lactide, and also is amenable in some instances to simultaneousracemization and melt crystallization of the S,S- and R,R-lactide.

The racemization should be conducted in the substantial absence of waterand other compounds that can react with lactide to form lactic acid,linear lactic acid oligomers, or other by-products. The starting lactidecomposition should contain no more than 50 milliequivalents/gram,preferably no more than 20 milliequivalents/gram, of water or otherhydroxyl-containing species, including lactic acid and oligomers oflactic acid. These can be removed from the starting lactide compositionusing various processes, if necessary before conducting the racemizationstep.

By conducting the racemization reaction at the temperatures describedabove, and in the presence of at most low levels of hydroxyl-containingspecies as just described, the racemization reaction proceeds withlittle of the lactide being converted to ring-opened species such aslactic acid or lactic acid oligomers. Typically, 20% or less of thestarting lactide is ring-opened during the racemization step, and it issometimes the case that 10% or less, 5% or less, 2% or less or even 1%or less of the starting lactide is ring-opened.

A catalyst is used in the racemization step so that reaction rates areincreased. Efficient catalysts include conjugate bases of carboxylicacids such as metal carboxylate salts of main group and transitionmetals. The carboxylic acid can vary widely, its choice being based onsuch attributes as basicity, solubility, and thermal stability. Otheruseful catalysts include metal salts of Group 5A and 6A acids, includingmetal salts of sulfonates (RSO₃—), sulfinates (RSO₂—), phosphonates[(RO)₂P(O)O—] and phosphinates [(RO)₂PO—], wherein R in each case ishydrocarbyl, preferably alkyl. Especially preferred catalysts arehomogeneous in the reaction mixture at the reaction temperature orimmobilized on a solid support.

Other suitable catalysts include non-nucleophilic bases such as acyclicand cyclic tertiary amines such as triethyl amine and1,4-diazabicyclo[2.2.2]octane, respectively. Still other usefulcatalysts include non-nucleophilic heterocyclics, such as pyridine andlutidine, as well as non-nucleophilic amidines such as1,8-diazabicyclo[5.4.0]undec-7-ene. Non-nucleophilic bases incombination with Lewis acids are also useful. Examples of these includethe combination bis-trimethylsilyl trifluoroacetamide and pyridine.

The catalyst should not contain reactive groups (such as activehydrogen-containing groups like hydroxyl, primary or secondary amino orthiol) that promote or engage in ring-opening reactions with thelactide. From 0.01 to about 5 parts by weight catalyst per 100 parts byweight of lactide is generally useful. A more preferred amount is from0.1 to 2.5 parts by weight of the catalyst.

The catalyst may be affixed to a support to facilitate separation of thecatalyst from the racemized lactide mixture.

As a result of the racemization, the proportions of S,S-, R,R- andmeso-lactide in the lactide become changed. If the predominate lactidespecies in the starting lactide is S,S-lactide, then, absent any removalof material during the racemization reaction, over time the proportionof S,S-lactide will decrease and the proportions of R,R- andmeso-lactide will increase. If the predominate lactide species in thestarting lactide is R,R-lactide, then, absent any removal of materialduring the racemization reaction, over time the proportion ofR,R-lactide will decrease and the proportions of S,S- and meso-lactidewill increase. In the preferred case, the predominate lactide species inthe starting lactide is meso-lactide, which will become less predominateover time as the proportions of S,S- and R,R-lactide increase. In allcases, the ratios of these lactide species will over time tend towards atemperature-dependent equilibrium, as discussed above, unless one ormore of the lactide forms is preferentially removed.

The process preferably includes a separation step, in which at least oneof the lactide species is separated from the other two species in theracemized mixture. In the preferred case, in which the starting materialis a meso-lactide stream, the separation will typically include theseparation of S,S-lactide, R,R-lactide or both S,S-lactide andR,R-lactide from the remaining meso-lactide.

The method by which the lactide species are separated is not critical,but the separation process preferably concentrates impurities in theremaining meso-lactide. In that manner, a racemized S,S- and/orR,R-lactide which is depleted in impurities, relative to the startinglactide composition and the remaining meso-lactide, can be removed fromthe product mixture.

“Depletion” in this case is with reference to the starting lactidecomposition; the weight ratio of impurities to the lactide content ofthe separated stream is lower than the weight ratio of the impurities tothe lactide content of the starting lactide composition. Depletion doesnot require complete removal of the impurities. This relationship can beexpressed by the inequality

$\begin{matrix}{1 > \frac{I_{{SS}\text{-}{RR}}/\left( {I_{{SS}\text{-}{RR}} + L_{{SS}\text{-}{RR}}} \right)}{I_{starting}/\left( {I_{starting} + L_{starting}} \right)}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$where I_(SS RR) represents the weight of the impurities in the racemizedS,S- and/or R,R-lactide stream separated from the reaction mixture,L_(SS-RR) represents the weight of lactide in the racemized S,S- and/orR,R-lactide stream separated from the reaction mixture, I_(starting)represents the weight of the impurities in the starting lactidecomposition and L_(starting) represents the weight of lactide in thestarting lactide composition. Preferably, the ratio in equation 1 isless than 0.1, more preferably less than 0.05 and even more preferablyless than 0.01. An advantage of the invention is that the racemized S,S-and R,R-lactide separated from the starting lactide stream can beproduced having a very low level of impurities, and so can bepolymerized with little or no additional purification.

One separation method of interest is a melt crystallization method, atleast in part because impurities tend to be excluded from the crystalsthat are formed in the crystallization step. Therefore, the meltcrystallization method provides a means for simultaneously recoveringlactic acid values from the starting lactide stream (in the form ofS,S-lactide and/or R,R-lactide), as well as removing impurities from therecovered lactic acid values.

Melt-crystallization has previously been described as a way forpurifying lactide. U.S. Pat. No. 6,310,218 describes meltcrystallization as a way to concentrate an S,S-lactide-rich stream, andto produce a purified racemic lactide. In general, melt crystallizationis performed on the racemized lactide stream by melting the lactidemixture, and then cooling the mixture so that the racemized S,S- and/orR,R-lactide crystallizes while leaving the bulk of the meso-lactide inthe melt phase.

The melt crystallization can be conducted batch-wise or continuously.Batch methods and zone methods for conducting a melt-crystallization asdescribed in U.S. Pat. No. 6,310,218 are suitable. Suitable apparatusfor performing industrial melt crystallizations include those described,for example, in U.S. Pat. Nos. 5,700,435, 5,338,519, 6,204,793 and6,145,340. In a melt crystallization method, crystals of the racemizedS,S- and R,R-lactide are formed, while leaving the meso-lactide in theform of a molten liquid. The solid crystals are then separated from themolten meso-lactide.

If the racemized mixture contains nearly equal quantities of S,S-lactideand R,R-lactide, the racemized S,S-lactide and R,R-lactide cancrystallize into a crystalline form known as racemic-lactide, which hasa freezing temperature of about 127° C. In that circumstance, the meltcrystallization can be conducted at a temperature above 56° C. (i.e.,the melting temperature of meso-lactide) and below 127° C. Thecrystallization temperature in that case preferably is from about 100°C. to 125° C., so that crystals tend to form slowly and in that mannermore completely exclude meso-lactide and impurities from theirdeveloping crystal structure.

When the racemized mixture contains significantly more S,S-lactide thanR,R-lactide, or vice versa, the racemized S,S-lactide and R,R-lactidewill not form a high-melting mixture. The racemized S,S-lactide andR,R-lactide instead will crystallize at or below the melting temperatureof S,S- and R,R-lactide, i.e., at about 97° C. In that case, thecrystallization temperature should be between 56° C. and 97° C. Apreferred crystallization temperature in that case is from 80 to 95° C.,to allow crystals to form slowly and in that manner more completelyexclude meso-lactide and impurities from their developing crystalstructure.

The racemization and melt crystallization steps can be performedsimultaneously and/or sequentially. Simultaneous racemization and meltcrystallization is more practical in cases in which the racemizedS,S-lactide and R,R-lactide form a mixture that freezes at about 127° C.If no such mixture forms, the melt crystallization temperature must bebelow 97° C., at which temperature the racemization rate becomes slow.Simultaneous racemization and melt crystallization can be done, in casesin which a high-melting mixture forms, by conducting the racemization ata temperature below 127° C. A preferred temperature is from 90 to 125°C., and a more preferred temperature is from 90 to 115° C. Using thisapproach, the racemized R,R-lactide and S,S-lactide that forms willcrystallize as racemic lactide (i.e., a high-melting mixture) as theracemization reaction proceeds. This approach has the advantages ofbeing easily adapted to continuous operation. When the starting lactideis a meso-lactide stream, this approach has the additional advantage ofcontinually removing R,R- and S,S-lactide from the system, therebyfavoring more R,R- and S,S-lactide production, as the chemicalequilibrium between the lactide forms is never reached.

In a sequential operation, the starting lactide composition is racemizedfirst, followed by a separate step of removing some or all of at leastone of the lactide species from the remaining species.

A hybrid process is possible, in which the starting lactide compositionis partially racemized at a higher temperature, followed by cooling theresulting mixture to a second, lower temperature at which racemizationcan continue simultaneously with the crystallization of racemized S,S-and R,R-lactide. This approach combines the advantages of higherracemization rates with simultaneous racemization and separation. Insuch a hybrid process, the second, lower temperature preferably is from100 to 125° C., more preferably from 105 to 110° C.

In any of the foregoing melt crystallization methods, the racemiclactide crystals or the S,S- and R,R-lactide crystals, as the case maybe, are removed from the molten meso-lactide using any convenientsolid-liquid separation method, such as filtering, decanting,centrifugation, plating the crystals out on a solid surface, and thelike.

Another way to separate the S,S- and/or R,R-lactide from meso-lactide isby distillation. Distillation can be conducted at atmospheric orsubatmospheric pressures. The racemization catalyst should be removedfrom the racemized mixture before the distillation step so that furtherracemization and formation of linear oligomers is minimized.Distillation produces a stream that is enriched in the racemized S,S-and R,R-lactide and a stream that is enriched in meso-lactide. Asbefore, impurities tend to become concentrated in the meso-lactidestream. The racemized S,S- and R,R-lactide are therefore obtained as astream that is depleted in impurities, as described before.

Yet another way to separate the S,S- and R,R-lactide from meso-lactideis by a solvent crystallization method. Suitable solvents include, forexample, a halogenated hydrocarbon such as chloroform or1,2-dichloroethane; an aliphatic or alicyclic ether such as diethyletheror tetrahydrofuran; an aromatic hydrocarbon such as toluene; a hinderedalcohol such as 2-propanol; and the like.

In certain embodiments of the invention, the starting lactidecomposition is a meso-lactide stream produced in an upstream step ofseparating a lactide mixture into the meso-lactide-enriched stream andan S,S- and R,R-lactide stream. In such a case, it is possible toperform step b) of the inventive process by recycling the racemizedproduct from step a) into that upstream separation step (or into someother step that is farther upstream). In such a case, it becomesunnecessary to separate racemized S,S- and R,R-lactide from theremaining meso-lactide prior to conducting the recycling step, as theseparation occurs in that upstream separation step.

It is also possible to preferentially separate out S,S- or R,R-lactidefrom the racemized lactide mixture. One way of doing this is bycrystallizing the mixture from either a melt or from a solvent, in thepresence of seed S,S- or R,R-lactide crystals. When seed S,S-lactidecrystals are present, it is possible to preferentially crystallizeS,S-lactide from the racemized lactide mixture, leaving R,R- andmeso-lactide behind. Conversely, R,R-lactide can be preferentiallyseparated by crystallizing in the presence of R,R-lactide seed crystals.The crystals in either case are enriched in S,S- or R,R-lactide, as thecase may be, and depleted in the other lactide species, relative to thecomposition of the racemized lactide mixture. Crystallization conditionshere are selected to promote slow crystal formation and growth, so thatthe unwanted lactide species do not rapidly crystallize together withthe desired species.

The S,S- and/or R,R-lactide that is separated from the racemized lactidemixture in the foregoing manner can be further purified if desired. Theracemized S,S- and/or R,R-lactide stream may contain various types ofimpurities, such as a certain amount of meso-lactide that becomesentrained in the material; small amounts linear lactic acid oligomers;small amounts of lactic acid that reforms during the racemization step,some residual quantity of other impurities, residual racemizationcatalyst, and the like. If significant amounts of any of these types ofimpurities are present, it may be necessary or desirable to remove thembefore the racemized S,S- and/or R,R-lactide is taken forpolymerization. In addition, residual racemization catalyst ispreferably removed from the racemized S,S- and/or R,R-lactide,particularly if it is to be taken to polymerization and/or recycled backinto the lactide production system. Various purification approaches maybe taken, including distillation, melt crystallization, solventcrystallization, absorption, extraction or like methods.

In one approach, the racemized S,S- and/or R,R-lactide is crystallized(if separated from the remaining meso-lactide by distillation) orrecrystallized (if separated from the remaining meso-lactide by meltcrystallization) one or more times. In a related approach, the racemizedS,S- and/or R,R-lactide can be separated from the remainingmeso-lactide, then crystallized, followed by “perfecting” the crystalsby heating them to a temperature just below their melting point toselectively exclude entrained impurities from the crystal structure.

In a third approach, crystals of the racemized S,S- and/or R,R-lactideare washed or extracted with a suitable solvent to remove entrainedimpurities. The racemization catalyst in particular may be removed ordeactivated this way, by washing with water or another deactivatingagent.

In a fourth approach, the racemized S,S- and/or R,R-lactide is distilledto further purify it. If such an approach is adopted, it is oftenconvenient to recycle the racemized S,S- and/or R,R-lactide back into anappropriate stage of the lactide production process, because theproduction process normally will already include one or more unitoperations which are designed to remove various sorts of impurities.This embodiment of the invention is further explained through referenceto the Figure. The Figure is a schematic diagram illustrating anembodiment of the process of the invention. The embodiment illustratedin the Figure illustrates various preferred or optional features. Figureis not intended to show specific engineering features or details,including the design of the various components shown. In addition,auxiliary equipment such as various valves, pumps, heating and coolingequipment, analytical, control devices and the like are not shown, butof course can be used as necessary or desirable.

In the Figure, lactic acid or lactic acid ester stream 5 containingwater or, less preferably, another solvent, is fed into prepolymerreactor 1. The lactic acid or lactic acid ester concentration in stream5 preferably is at least 60% by weight, and may be as high as 95% byweight, preferably as high as 90% by weight. Lactic acid may be obtainedfrom a fermentation broth, which is preferably concentrated to withinthe aforementioned ranges in an upstream process step which is not shownin the Figure. The starting material is heated in prepolymer reactor 1to cause the lactic acid or lactic acid ester to condense to form aprepolymer as described before. Most of the water, solvent (if any) andcondensation by-products are removed from prepolymer reactor 1separately from the prepolymer.

Prepolymer stream 6 is removed from prepolymer reactor 1 and transferredto lactide reactor 2, where it is depolymerized to form lactide. Lactidereactor 2 is essentially an evaporator, and can be of many types asdescribed in WO 95/09879. Examples of suitable lactide reactors include,for example, forced circulation, short path or short tube, long-tubevertical, long-tube horizontal, falling film, agitated thin-film anddisk evaporators. Film-generating evaporators, especially falling filmand agitated falling film evaporators as described in WO 95/9879, areespecially preferred. Various types of staged reactors are alsosuitable. Lactide reactor 2 is preferably operated at a pressure of fromabout 1 to about 100 mm Hg, preferably from about 2 to about 60 mm Hg.An elevated temperature, preferably from about 180 to 300° C. and morepreferably from 180 to 250° C., is used.

The depolymerization reaction in lactide reactor 2 is usually catalyzed.As shown, catalyst is introduced to prepolymer stream 6 upstream oflactide reactor 2, through catalyst stream 18. It is also possible tointroduce catalyst stream 18 directly into lactide reactor 2.

Crude lactide and a bottoms mixture are formed in lactide reactor 2. Thebottoms mainly include high boiling materials and higher oligomers oflactic acid. The bottoms are withdrawn as a bottoms stream (not shown).

Crude lactide formed in lactide reactor 2 is withdrawn as stream 8 andtransferred to distillation column 3. In the embodiment shown, the crudelactide is distilled in three stages, in first distillation column 3,second distillation 4 and third distillation column 20, respectively. Itis possible in principle at least to carry out the entire distillationin a single column or only two distillation columns.

In the embodiment shown, crude lactide stream 8 is introduced into firstdistillation column 3, where it is separated into partially purifiedlactide stream 10 and an overhead stream (not shown). A bottoms stream(not illustrated) also may be withdrawn from first distillation column3. The overhead stream contains some lactide together with most of thewater and lactic acid. Partially purified lactide stream 10 containslactide and most of the impurities that have relative volatilities offrom 1.001 to 1.5 relative to S,S- or R,R-lactide when distilled from alactide matrix (“intermediate-boiling impurities”). It is normallysubstantially depleted of water and lower-boiling impurities, althoughsome may remain.

In the embodiment shown, partially purified lactide stream 10 istransferred to second distillation column 4, where lactide is separatedfrom higher-boiling impurities such as linear lactic acid oligomers.This produces purified lactide stream 25 and a bottoms stream (notshown). Purified lactide stream 25 contains intermediate-boilingimpurities as described before. Some volatiles (mainly water and lacticacid) may in addition be removed from second distillation column 4(through a top outlet, not shown).

In the embodiment shown in Figure, purified lactide stream 25 istransferred to third distillation column 20, where the meso-lactide isseparated from S,S- and R,R-lactide. As shown, this produces a purifiedS,S-lactide/R,R-lactide stream 13, which is withdrawn from near thebottom of third distillation column 20, and a meso-lactide stream 14,which is withdrawn from near the top of third distillation column 20.Meso-lactide stream 14 should contain no more than 50, and preferably nomore than 20, milliequivalents/gram of hydroxyl-containing species suchas water, lactic acid and lactic acid oligomers. A bottoms stream (notshown) may also be removed from third distillation column 20. PurifiedS,S-lactide/R,R-lactide stream 13 is transferred to polymerization unit23, where it is polymerized to form polylactide.

Meso-lactide stream 14 is taken to racemization unit 17, where at leasta portion of the meso-lactide in stream 14 is racemized to formracemized S,S- and R,R-lactide. Racemized S,S- and R,R-lactide isremoved from racemization unit 17 through line 15. The S,S- andR,R-lactide removed through line 15 and line 16 then can be recycledthough any of all of lines 16A, 16B, 16C or 16D, optionally after beingfurther purified in optional purification unit 19. Purification unit 19can be any unit operation or unit operations such as those describedabove in which impurities are removed from the racemized S,S- andR,R-lactide.

If racemized S,S- and R,R-lactide stream 15 or 16 is sufficiently pure,it can be recycled through line 16A directly into polymerization unit23, where it can be polymerized by itself to form an amorphous grade ofpolylactide. It is also possible to polymerize a stream containing bothS,S- and R,R-lactide, such as a racemic lactide stream, to produce asemi-crystalline polymer having a crystalline melting temperature in therange of from about 145° C. to 210° C. or more, using certain salen- andhomosalen-aluminum complexes, as described by Nomura et al., in“Stereoselective ring-opening polymerization of a racemic lactide byusing achiral salen- and homosalen-aluminum complexes”, Chem. Eur. J.2007, 13, 4433-4451.

More typically, the S,S- and/or R,R-lactide will be polymerized as ablend with a S,S-lactide/R,R-lactide stream, such as theS,S-lactide/R,R-lactide stream entering polymerization unit 23 throughline 13. In the usual process, the ratios of S,S-lactide/R,R-lactide instream 13 and the racemized S,S- and R,R-lactide recycle stream 16A willbe selected such that the resulting blend contains at least 90%,preferably from 92 to 99.5% by weight of one of S,S-lactide orR,R-lactide, and no more than 10%, preferably from 0.5 to 8%, by weightof the other. The lactide blend so produced can then be polymerized toproduce a semi-crystalline polylactide resin.

If the racemized S,S- and R,R-lactide stream 15 or 16 still containssignificant quantities of meso-lactide, but otherwise is purified enoughto be polymerized, it may instead be recycled through line 16D back intothird distillation column 20, in which the crude lactide entering fromline 25 is separated into a meso-lactide stream 14 and S,S- andR,R-lactide stream 13. The racemization catalyst should be removed ordeactivated in this case prior to recycling. The recycled racemized S,S-and R,R-lactide will then exit third distillation column through line13, and most if not all of the meso-lactide will again be sent outthrough line 14 for racemization.

If racemized S,S- and R,R-lactide stream 15 or 16 contains significantquantities of other volatile impurities, it can be recycled throughlines 16B and/or 16C to either or both of first distillation column 3and second distillation column 4, to be purified together with the crudelactide that is produced in lactide reactor 2. Again, in this case, theracemization catalyst should be removed or deactivated.

The enriched meso-lactide stream (such as stream 21 in the Figure) thatremains from the racemized mixture can be handled in various ways. Itcan be discarded or used in low value applications, an advantage of thisinvention in this case being that the mass of this stream is reducedrelative to conventional processes and therefore there are fewer wastedlactic acid equivalents and lower disposal costs. Alternatively, theintermediate-boiling impurities can be removed from this stream, and themeso-lactide then can be recycled back into the lactide productionprocess. This approach allows more of the lactic acid equivalents to berecovered. It is also possible to recycle it together with the racemizedS,S- and R,R-lactide stream 15 or 16.

One method of removing intermediate-boiling impurities from meso-lactideis through an extraction and/or chemical treatment method. In general,methods of this type include (a) extraction with a solvent in whicheither the meso-lactide or the intermediate-boiling impurities (or somesubset thereof), but not both, have a good solubility; (b) convertingthe meso-lactide and/or the intermediate-boiling impurities (or somesubset thereof) to different chemical species which are more easilyseparated, and then separating the impurities or their reaction productsfrom the meso-lactide or its reaction products, as the case may be. Inthe latter case, the separation may be done by a further distillation,an extraction process, a filtration process (if a solid chemical speciesis formed), or other separation technique, depending of course on theparticular chemical species that are formed in a given case.

Succinic anhydride is often a large component of theintermediate-boiling impurities. Succinic anhydride can be separatedfrom meso-lactide by washing the meso-lactide stream with a weakly basicaqueous phase. A suitable pH of the washing solution is in the range offrom about 7.2 to about 11. This is believed to hydrolyze the succinicanhydride to form succinic acid and to neutralize the succinic acid.Some of the lactide may also be hydrolyzed to form mainly linearoligomers and possibly some lactic acid. The hydrolyzed and neutralizedsuccinic acid is much more soluble in the aqueous phase than in thelactide phase, and will partition to the aqueous phase. The aqueous andorganic phases are then separated to form a washed meso-lactide streamhaving a reduced level of intermediate-boiling impurities. The washedmeso-lactide stream can be recycled into the prepolymer-forming step, orfurther upstream in the process.

If intermediate-boiling impurities are removed from the meso-lactidewithout hydrolyzing the meso-lactide to form linear oligomers or lacticacid, the meso-lactide can be recycled into any convenient portion ofthe process, including, for example, the prepolymerization step (i.e.,into prepolymerization reactor 1 in the Figure), the depolymerizationstep (i.e., into lactide reactor 2 in the Figure), or a lactidedistillation step (i.e., into first distillation column 3 and/or seconddistillation column 4 in the Figure). If necessary, the racemizationcatalyst may be removed or deactivated before the meso-lactide isrecycled.

S,S- and/or R,R-lactide obtained from the process can be polymerized, ifnecessary after further purification as just described.

The following examples are provided to illustrate the invention, but arenot intended to limit the scope thereof. All parts and percentages areby weight unless otherwise indicated.

EXAMPLES 1-3

Crude lactide is prepared by polymerizing S-lactic acid to form aprepolymer, and then depolymerizing the prepolymer to produce a crudelactide vapor stream. The crude lactide is then distilled in multiplesteps to remove volatiles, and to produce an S,S-lactide stream (whichcontains a small amount of R,R-lactide) and a meso-lactide stream. Themeso-lactide stream contains about 6.8 moles of meso-lactide/kilogramand about 0.2 moles of S,S-lactide/kilogram, and also containsintermediate-boiling impurities. It contains less than 50milliequivalents/gram of hydroxyl-containing impurities. Themeso-lactide stream is collected and cooled.

About 500 mL of the meso-lactide stream is melted in an oil bath.Na-ethylhexanoate catalyst is added at a 0.05% level. The mixture isheated in a stirred Parr reactor at 160° C. for 15 hours. Samples aretaken periodically to measure the concentration of S,S-lactide,R,R-lactide and meso-lactide in the mixture.

As the reaction proceeds, the concentration of meso-lactide fallssteadily, and S,S-lactide and R,R-lactide are produced at essentiallyequal rates. An equilibriva is established after about 15 hours reactiontime. The equilibrium mixture contains about 36 mole percent each ofS,S- and R,R-lactide, and about 28 mole percent meso-lactide. Thereaction mixture contains less than 0.5% by weight of linear oligomersof lactic acid.

The reaction mixture is then cooled to a temperature of about 115-125°C. Racemic lactide crystals slowly form at this temperature to create asolid phase containing essentially all of the S,S- and R,R-lactide, anda liquid phase that contains meso-lactide, some S,S- and R,R-lactide,and essentially all of the intermediate-boiling impurities. The racemiclactide crystals contain less than 0.6% of linear oligomers of lacticacid.

When this example is repeated using a 140° C. racemization temperature,the meso-lactide takes longer to reach an equilibrium mixture, and theequilibrium mixture is weighted slightly more towards the S,S- andR,R-lactide. Linear oligomer content is slightly lower in the racemizedproduct.

When the experiment is repeated at 180° C., the racemization rate issignificantly faster, but the equilibrium is weighted slightly moretowards meso-lactide, and linear oligomers form about 5% of the product.

EXAMPLE 4

56 parts by weight of a meso-lactide stream are melted and combined with0.14 parts of 1,4-diazabicyclo[2.2.2]octane. The starting meso-lactidestream contains 98 mole percent meso-lactide, 2 mole percent S,S-lactideand less than 50 milliequivalents/gram of hydroxyl-containingimpurities. The lactide/catalyst mixture is charged into a Sulzer staticcrystallizer, wherein it is maintained at 95° C. for 15.5 hours. Duringthis time, the meso-lactide racemizes to form a mixture of S,S-, R,R-and meso-lactides, and a portion of the racemized-lactide forms acrystalline solid. After 15.5 hours, the remaining liquid (18 parts byweight) is removed from the crystallizer and analyzed. It is found tocontain approximately equal quantities of R,R- and S,S-lactide, and 24.8mole percent meso-lactide.

The crystallized material remaining in the static crystallizer is thenheated gradually over a period of two to three hours. During this time,the crystals “sweat”, i.e., release entrained material. This entrainedmaterial is captured, weighed and analyzed. 6.5 parts by weight ofmaterial is released during this “sweating” step; that material containsapproximately equal quantities of R,R- and S,S-lactide and about 25 molepercent of meso-lactide.

The remaining crystals (30.1 parts by weight) contain about 43.3 partsby weight each of R,R- and S,S-lactide, and about 13.5 parts by weightof meso-lactide. These crystals are melted and transferred into thetubes of a falling film crystallizer. The lactide-containing tubes arethen cooled to crystallize the lactide. As cooling continues, lactidecrystals plate out on the internal surfaces of the tubes. The lactidecrystals conduct heat poorly, and eventually heat transfer is slowedenough that crystal formation ceases. At that point, the remainingliquid is drained. 6.8 parts of liquid are removed. That liquid contains45.5 mole percent meso-lactide and approximately equal quantities ofR,R- and S,S-lactide. The 1,4-diazabicyclo[2.2.2]octane catalyst becomesconcentrated in the remaining liquid.

The lactide crystals that remain in the lactide are “sweated” as before,and release 1.6 parts of a lactide that contains 38.5 mole percentmeso-lactide and approximately equal amounts of R,R- and S,S-lactide.

About 21.6 parts of crystals remain. These contain about 5.0 molepercent meso-lactide and approximately equal amounts of R,R- andS,S-lactide, and are purified sufficiently to be used in apolymerization process. These crystals contain about 0.02 weight percentof the 1,4-diazabicyclo[2.2.2]octane catalyst.

To purify the crystals further, crystals are melted and again processedthrough a falling film crystallizer as before. After crystallizationstops due to poor heat transfer, 5.7 parts of a liquid residue remains.This is drained; it contains about 17.9 mole percent of meso-lactide andagain approximately equal quantities of R,R- and S,S-lactide. Thecrystals are “sweated” again, releasing 3.2 parts of a liquid. Thecrystals that remain after this second recrystallization are melted andrecovered. Yield is 12.4 parts of a lactide mixture that contains 0.5mole percent meso-lactide, 49.8 mole percent R,R-lactide and 49.7 molepercent S,S-lactide.

EXAMPLE 5

About 900 mL of a meso-lactide stream is melted in an oil bath.1,4-Diazabicyclo[2.2.2]octane catalyst is added at a 0.5% level. Themixture is heated unstirred in a sealed vessel at 120° C. for 4 hours.Samples are taken periodically to measure the concentration ofS,S-lactide, R,R-lactide and meso-lactide in the mixture.

As the reaction proceeds, the concentration of meso-lactide fallssteadily, and S,S-lactide and R,R-lactide are produced at essentiallyequal rates. An equilibriva is established after 1 hour reaction time.The equilibrium mixture contains about 42 mole percent each of S,S- andR,R-lactide, and about 16 mole percent meso-lactide. The reactionmixture contains 15% by weight of linear oligomers of lactic acid.

The racemized mixture is cooled until it solidifies. The solidifiedlactide is dissolved in 1.2 L of toluene (approx 1 g per mL) by heatingto 75° C. in a two-neck 3 L flask. The solution is removed from the heatsource and allowed to cool over night to deposit crystals. Thecrystallized lactide is isolated by filtration using a 600 mL glassfrit. The recovered lactide, 618.3 g, is dissolved in 1.2 L ofisopropanol by heating to 75° C. in a two-neck 3 L flask. The solutionis removed from the heat source and allowed to cool for 5 hours todeposit crystals. The crystallized lactide is isolated by filtrationusing a 600 mL glass frit. The recovered lactide is dried in a vacuumoven at 40° C. overnight. The overall recovery of racemic-lactide is60.3 percent based starting lactide. The lactide crystals contain 0.2mole percent meso-lactide and nearly equal amounts of R,R- andS,S-lactide.

EXAMPLE 6

When Example 5 is repeated using 0.05% catalyst, the system does notreach an equilibrium mixture within 10 hours. At this point, the samplecontains 44 mole percent meso-lactide, 30 mole percent S,S-lactide and24 mole percent R,R-lactide. The reaction mixture contains 5% by weightof linear oligomers of lactic acid. The overall recovery ofracemic-lactide is 56.7 percent based starting lactide. The lactidecrystals contain 0.2 mole percent meso-lactide and nearly equal amountsof R,R- and S,S-lactide.

What is claimed is:
 1. A process for recovering lactic acid values froma starting lactide composition, comprising a racemization step ofsubjecting a starting lactide composition to a temperature of up to 170°C. in the presence of a racemization catalyst for a time sufficient toracemize at least a portion of the lactide in the starting lactidecomposition to form a racemized lactide mixture that containsmeso-lactide, S,S-lactide and R,R-lactide in relative proportionsdifferent than in the starting lactide composition, wherein theracemization catalyst is triethyl amine, 1,4-diazabicyclo[2.2.2]octane,pyridine, lutidine, 1,8-diazabicyclo[5.4.0]undec-7-ene or sodiumethylhexanoate.
 2. The process of claim 1, wherein the starting lactidecomposition contains no more than 50 milliequivalents/gram ofhydroxyl-containing compounds.
 3. The process of claim 2, wherein theracemization step is conducted at a temperature of from 135 to 170° C.4. The process of claim 2, wherein the racemization step is conducted ata temperature from 90 to 125° C.
 5. The process of claim 2, wherein thestarting lactide composition is a meso-lactide stream that contains atleast 60% by weight meso-lactide.
 6. The process of any of claim 5,wherein the meso-lactide stream is produced as a residue stream in amelt crystallization of a crude lactide mixture.
 7. A process of claim5, wherein the meso-lactide stream is produced by fractionallydistilling a crude lactide mixture to form the starting meso-lactidestream and an S,S- and R,R-lactide stream.
 8. The process of claim 7wherein the crude lactide mixture is prepared by a process that includesthe steps of 1) forming a low molecular weight poly(lactic acid); and 2)depolymerizing the low molecular weight poly(lactic acid) to form thecrude lactide mixture.
 9. The process of claim 8 further comprising step3), removing impurities from the crude lactide mixture prior to orsimultaneously with the fractional distillation step.
 10. The process ofclaim 2, further comprising a separation step of separating theracemized lactide mixture to obtain at least one lactide product that isenriched in S,S-lactide, R,R-lactide or meso-lactide, or any twothereof, relative to the racemized lactide mixture.
 11. The process ofclaim 10, wherein a product enriched in S,S-lactide and R,R-lactide,relative to the racemized lactide mixture, is separated from racemizedlactide mixture.
 12. The process of claim 11, wherein the productenriched in S,S-lactide and R,R-lactide is depleted in impuritiesrelative to the starting lactide composition.
 13. The process of claim10, wherein at least a portion of the racemization step is performedsimultaneously with the separation step.
 14. The process of claim 10,wherein the separation step includes a melt crystallization step. 15.The process of claim 14, wherein the melt crystallization step isconducted at a temperature of from 90 to 125° C., and racemic lactidecrystals form during the melt crystallization step.
 16. The process ofclaim 10 further comprising polymerizing racemized S,S-lactide andR,R-lactide obtained in the separation step.
 17. The process of claim 16wherein the racemized S,S-lactide and R,R-lactide obtained in theseparation step are polymerized in the presence of an achiral salen- orhomosalen-aluminum complex.
 18. The process of claim 10 whereinracemized S,S-lactide and R,R-lactide obtained from the separation stepis recycled into at least one of the fractional distillation step, step1), step 2) or step 3).
 19. The process of claim 2 wherein, after theracemization step, the racemized lactide mixture contains no more than10% by weight of lactic acid and lactic acid oligomers.